In the ongoing effort to reduce the weight and increase the crash energy absorbing characteristics of structural members in automobile bodies and the like, attention is being given to the use of hollow tubular laminated composite bodies. Typically, such bodies are cylindrical tubes that are round, square or rectangular in cross section and are made up of several plies or layers of woven or nonwoven reinforcing fabric that is embedded in or infiltrated with a solid matrix of a thermoset or thermoplastic resin material. The reinforcing fabric is suitably made of oriented or randomly arranged fibers of materials such as carbon, aramid filaments available commercially, for example, under the trade name Kevlar, glass fibers and mixtures or hybrids of such fibers. Examples of suitable matrix materials are epoxy and polyester resins.
In automotive applications, though sometimes the tube is straight over its entire length, in general, it has a straight section that is intended to resist an impact acting on the end of the tube along its longitudinal axis. For example, the tube may be aligned as a rail in the fore/aft direction of the automobile and intended to absorb impacts that are imposed on the bumper system of the vehicle.
The tube is intended to absorb the energy of a crash by undergoing a progressive crush, beginning at the lead end of the tube. Observed desirable failure modes include progressive longitudinal tearing, peeling or splitting, and accordion-like buckling of the structural member in the direction of crash. This is the intended dominant failure mode of the laminated composite tube. The challenge with respect to these strong laminated structural bodies is to provide a crush initiator that leads to a stable progressive crush of the member beginning at the lead end and progressing stably along its length so that the tube does not fail prematurely and catastrophically at a point further down its length, either locally due to high compressive forces or globally due to column buckling.
Previous attempts to provide axial crush initiation at the lead end of these composite tube structures have included use of a plug initiator or trigger that is inserted into the end of the tube. The plug, typically a specially-designed metal member, progressively cuts or slices its way down the tube as a crush force is applied. Other approaches of initiating crush at the lead end have been to bevel the edges of the four sides at the end of the tube or to cut back each of the four sides at the end of the tube into rounded, tulip-like configurations (see FIG. 2B described below). The bevels in many cases have failed to suitably reduce the crush initiation loads, which has either led to instantaneous, catastrophic failure of the tube structure or the transmission of excessive forces to the occupant compartment rather than initiating a desired progressive crush. The tulipped end has experienced several similar deficiencies and in addition has impeded the attachment of another structural member such as a bumper. The use of a plug-like initiator adds considerable mass to the structure and also may dramatically reduce the force level during a stable progressive crush so that the member does not absorb the amount of energy that it was designed to absorb.
Accordingly, there remains a need to better enable a laminated composite tubular crash energy absorber member to better reduce the magnitude of crush initiation forces and to absorb crash energy in a progressive manner.